That exact same thing happened to me a few months ago!! My doctor had no idea what it was. He even tested it for herpes to see if it was a cold sore. It wasn't and he couldn't tell me what it was. He put me on antibiotics though and it went away finally. Glad to know what it is called.
You may have developed a traumatic cyst--a fluid-filled sac brought on by biting. This isn't serious, but won't go away on its own. A dermatologist can diagnose this for you and probably treat it very simply if necessary using electrocautery. (Of course, it may not be a cyst, in which case medical treatment with creams and such may do the trick.)
Best.
Dr. Rockoff